A crusader against synthetic cannabis has seen firsthand the "horrendous" impact of banning it, after her son was charged with possessing the real thing.

Carterton district councillor Jill Greathead, who is also chairwoman of the Wairarapa Psychoactive Substances Working Party, was a vocal critic of legal highs legislation, and welcomed a recent amendment banning the substances until they can be proven safe.

But on Thursday in the Masterton District Court she saw another side of criminalisation when her 17-year-old son was charged with possessing cannabis.

"He's one of the lucky ones, he knows he's loved, he's got supportive parents. But if you, one, suffer addiction and, two, don't have a supportive family environment, it's going to be horrendous - you can kiss them goodbye, they'll fall into the system and it's just a deep black hole," she said.

Greathead said she was not making excuses for him and accepted he had done wrong, but took issue with a 17-year-old being treated as a criminal.

"He still can't vote, drink alcohol, or, before they changed it, buy legal psychoactive substances, but they treat him as an adult."

She said her son was at a house on Sunday, May 4, where cannabis was found by police. While he admitted smoking some, it wasn't his and nor was it found on him.

He went to the police station to give information the next day and instead found himself arrested, charged, taken to an interview room and read his rights while being videoed.

"[He] didn't have a bloody clue . . . he's only 17, he should have had a lawyer present, or at least an adult."

Greathead called for a law change to oblige police to call a supportive adult when anyone aged up to 19 was charged.

Police told her that, under the Privacy Act, they could not tell her of the charge, which meant her son, who was frightened and angry from the experience, had to tell her himself. He was so ashamed to do so that he became suicidal, she said.

"[He] had been crying for two hours, he wanted to kill himself. I've never seen him in such an emotional state, and that process did that to a human being."

She insisted she was not anti-police, but said the police and court process was too adversarial, and that the legal high act's amendment would put more young people in her son's shoes.

"That's not the process to deal with [teen] addicts. We can say they're goners, we'll have ruined, we'll have destroyed human beings . . . Before you put this pressure on our children, at least [parents] should be given the opportunity of turning up."

Convicting young people for possessing cannabis or once-legal highs could stop them travelling or getting jobs, make them anti-authority and "cut them off at the legs" before their lives had properly begun, she said.